Electromagnetic actuators are well known. In many applications, the output force of the actuator is controlled by and is a function of an electrical control or command signal, and as such can be used in a variety of applications. One type of electromagnetic actuator is the "linear" actuator wherein output force is linearly proportional to the input electrical current.
One type of linear actuator is shown in Prior Art FIG. 1. More particularly, FIG. 1 shows a prior art actuator 10 which is commercially available from Northern Magnetics of Van Nuys, Calif. The prior art actuator 10 includes magnetic flux conductive cylindrical case 12 having an inner wall 14 extending between a first end 16 and second end 18 of the case 12. An electrical current conductive coil 20 is wound on a nonmagnetic material coil carrier 21. The carrier 21 is coaxially secured within the case 12 with the windings of the coil 20 being intermediate the carrier 21 and the inner wall 14. The windings are made from thin copper wire.
A core assembly 19 includes an axially polarized cylindrical magnet 22, a first and a second pole piece 28, 30, and a cylindrical rod 32. The axially polarized cylindrical magnet 22 has a first magnetic pole at its first end 24 and a second opposite magnetic pole at its second end 26. The first disc shaped magnetic flux conductive material pole piece 28 is attached to the first end 24 of the permanent magnet 22. The second magnetic flux conductive material pole piece 30 is connected to the second end 26 of the permanent magnet 22.
The permanent magnet 22 and the first and second pole pieces 28, 30 are coaxially mounted to the cylindrical rod 32 which, in turn, is coaxially received by end caps 34, 36 in axial slideable engagement. Each end cap 34, 36 is attached to the cylindrical case 12. The rod 32 is received in slideable engagement in coaxial bores 38, 40 in each respective end cap 34, 36. The cylindrical rod 32 and end caps 34, 36 are of nonmagnetic material. The actuator 10 is one type of moving core actuator wherein the entire core assembly 19, including the magnet 22, pole pieces 28, 30, and rod 32, moves in order to perform the desired function of the actuator. As a result the moving mass of the core assembly 19 is relatively heavy.
The first pole piece 28 provides flux in a radial first direction across the coil 20 and the second pole piece 30 provides flux in the opposite radial direction across the coil 20. Ideally, the flux is confined to the case 12 in the axial section between the present position of the first pole piece 28 and the second pole piece 30. Thus, if current is put into the coil 20 at its midpoint 42, with the current return being at a first end 44 and a second end 46, with each end 44, 46 connected in common, then the current flux cross product with each pole piece 28, 30 will be additive. Alternatively, the coil 20 of the prior art actuator 10 may also be counterwound at either side of the midpoint 42 as set forth in the '158 Patent.
One problem with the prior art actuator is that ideal flux utilization does not exist. Since the magnet 22 is axially polarized, there will be leakage of the flux from the first pole piece 28 to the second pole piece 30 at the point they are attached to the rod 32 through the center bore of the rod 32. Accordingly, not all the available flux from the magnet 22 is being utilized to provide radial flux in confined axial regions of the coil 20. This flux loss reduces the total output power available from the actuator 10.
Furthermore, the hard magnetic material used in the prior art actuator limits the flux density through the actuator core. The magnetic material used in the prior art actuator carries a maximum flux density of 1.3 Tesla. In comparison, the actuator of the present invention utilizes cobalt iron or iron, which carries a maximim flux density of 2.4 Tesla. Therefore, for a given volume, the actuator of the present invention carries approximately 84% more flux density than the prior art actuator. Accordingly, the actuator of the present invention provides more force than the prior art actuator with the same amount of energy consumption.
Another limitation on the total output force available from the prior art actuator 10 is due to the coil carrier 21 being disposed between the pole pieces 28, 30 and the coil 20. The nonmagnetic material carrier 21 enlarges the gap in which the flux is confined, thereby reducing field strength.
A limitation on the dynamic output force capability of the prior art actuator is that the entire core assembly, including the magnet, pole pieces and rod, must be accelerated in order for the actuator to perform its function. As a result, a substantial portion of the actuator's energy is used to move the core assembly's relatively heavy weight. For a fixed force, the stroke of the actuator is inversely proportional to the actuator's moving mass. Therefore, for a fixed maximum force, the actuator's stroke and frequency capabilities are limited.